This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2008-215594 filed in Japan on Aug. 25, 2008, the entire contents of which are herein incorporated by reference.
The present invention relates to a sheet transport apparatus in which the presence or absence of a sheet is detected by an optical sensor at a curved portion in a sheet transport path, which is curved at an upstream side in the sheet transportation direction of a registration roller that transports sheets, and driving of the registration roller is stopped after an elapse of a predetermined transportation time from detection by the optical sensor of sheet absence following sheet presence of a sheet transported by the registration roller; and an image forming apparatus provided with the sheet transport apparatus.
For example, in electrophotographic image forming apparatuses, generally, an electrostatic latent image is formed on the surface of an image carrier such as a photosensitive drum; a toner image is formed on the surface of the image carrier by developing the electrostatic latent image on the surface of the image carrier using a developer; the toner image is transferred from the image carrier to a sheet such as plain paper or an OHP sheet while the sheet sandwiched in a nip region between the image carrier and a transfer member is being transported; and the sheet is heated and pressed, thereby fixing the toner image on the sheet. There is also a case where the toner image is transferred temporarily from the image carrier to an intermediate transfer body, and the toner image is fixed on the sheet after transferring the toner image on the intermediate transfer body from the intermediate transfer body to the sheet.
In such image forming apparatuses, a sheet transport apparatus is incorporated, and the sheet is drawn out from a paper feeding unit such as a paper feeding tray and transported by such a sheet transport apparatus. Although it is more preferable that the sheet transport path is straight in the sheet transport apparatus, in many cases, the sheet transport path cannot be kept straight and is curved at a plurality of positions because of the positional relationship between constituent members such as the paper feeding tray and the photosensitive drum.
Furthermore, in the transport path of the sheet transport apparatus, a registration roller (also called PS (Paper Stop) roller) is provided at an upstream side in the sheet transportation direction of the nip region between the image carrier and the transfer member or the nip region between the intermediate transfer body and the transfer member, and the leading edge (downstream side edge in the sheet transportation direction) of the sheet is put against the registration roller that is in a stopped state so as to flex the sheet so that the leading edge of the sheet is arranged in parallel to the registration roller due to the flexibility of the sheet, thereby transporting the sheet to the nip region by the registration roller. This prevents the sheet from obliquely passing through the nip region, avoiding oblique transfer of the toner image onto the sheet.
Meanwhile, after a sheet has passed through the registration roller, it is necessary that rotational driving of the registration roller is stopped before the next sheet arrives at the registration roller to put the leading edge of the next sheet against the registration roller so that the sheet flexes, but, in many cases, the rotational driving of the registration roller is stopped by the detection of the presence or absence of a sheet at an upstream side of the registration roller in the sheet transportation direction. An optical sensor is sometimes used as a means for detecting the presence or absence of a sheet. In a sheet transport apparatus using an optical sensor, rotational driving of the registration roller is temporarily stopped after an elapse of a predetermined transportation time from detection of the trailing edge (upstream side edge in the sheet transportation direction) of the sheet by an optical sensor so as to be ready to transport the next sheet. The predetermined transportation time is at least the time for the trailing edge of the sheet to pass through a post-detection transportation distance, i.e., from a sheet trailing edge detection point to the registration roller, and can be obtained in advance from the post-detection transportation distance and the sheet transportation speed.
Meanwhile, when the presence or absence of a sheet transported by the registration roller is detected by an optical sensor in a sheet transport apparatus provided with a sheet transport path that is curved at an upstream side with respect to the registration roller in the sheet transportation direction, variation in sheet characteristics such as sheet stiffness and optical characteristics is not preferable in that the time of the detection of the sheet trailing edge by the optical sensor may differ. This is further described with reference to FIGS. 6A and 6B, using an example where a reflective optical sensor that detects the presence or absence of a sheet based on reflected light, that is light applied to a sheet being transported and reflected from the sheet, is used as the optical sensor.
FIGS. 6A and 6B are diagrams illustrating the inconveniences in the case where a reflective optical sensor 102 is used in a conventional sheet transport apparatus provided with a sheet transport path S that is curved at an upstream side of a registration roller 101 in the sheet transportation direction. FIG. 6A illustrates a detection state by the reflective optical sensor 102 when a plain paper Q1 is transported, and FIG. 6B illustrates a detection state by the reflective optical sensor 102 when an OHP sheet Q2 which is stiffer than the plain paper is transported.
The conventional sheet transport apparatus shown in FIGS. 6A and 6B is provided with the registration roller 101 that transports the sheets Q1 and Q2, the sheet transport path S including a curved portion 71 that is curved at an upstream side of the registration roller 101 in the sheet transportation direction (the direction of arrow Xd in the figure), and the reflective optical sensor 102 that detects the presence or absence of the sheets Q1 and Q2 at the curved portion 71 of the sheet transport path S; and rotational driving of the registration roller 101 is temporarily stopped after an elapse of a predetermined transportation time from the detection by the reflective optical sensor 102 of sheet absence following sheet presence of the sheets Q1 and Q2 transported by the registration roller 101 so as to be ready to transport the next sheet. In this sheet transport apparatus, trailing edges Q1d and Q2d of the sheets Q1 and Q2 are determined by monitoring the non-detection of reflected light R1 and R2 that is incident on the reflective optical sensor 102.
The registration roller 101 is rotationally driven around the axis in the sheet transportation direction (direction of arrow X in the figure) while sandwiching the sheets Q1 and Q2 between the registration roller 101 and a facing roller (here, an idler roller) 105 facing the registration roller 101, so as to transport the sheets Q1 and Q2.
The reflective optical sensor 102 is provided outside the curved portion 71 of the sheet transport path S, and is made up of a light-emitting unit 102a including a light-emitting element that applies outgoing light L onto the sheets Q1 and Q2 transported by the registration roller 101, and a light-receiving unit 102b including a light-receiving element that receives the reflected light R1 and R2 reflected from the sheets Q1 and Q2.
In the conventional sheet transport apparatus thus configured, there is a difference in detection time by the sensor 102, i.e., the detection time by the sensor 102 determined by the trailing edge Q1d of the plain paper Q1 passing through a light-application region of the reflective optical sensor 102 during the transportation of the plain paper Q1 (as shown in FIG. 6A), and the detection time by the sensor 102 determined by the trailing edge Q2d of the OHP sheet Q2 passing through the light-application region of the reflective optical sensor 102 during the transportation of the OHP sheet Q2 (as shown in FIG. 6B).
To be specific, the incoming direction of the reflected light R1, that is outgoing light L applied by the light-emitting unit 102a to the plain paper Q1 toward the trailing edge Q1d and reflected from the plain paper Q1, to the light-receiving unit 102b (ref: FIG. 6A) is substantially constant, whereas the outgoing light L applied from the light-emitting unit 102a onto the OHP sheet Q2 toward the trailing edge Q2d (ref: FIG. 6B) may be easily transmitted through the OHP sheet Q2 (ref: Ld in FIG. 6B), and the incoming direction of the reflected light R2 reflected from the OHP sheet Q2 to the light-receiving unit 102b may greatly deviate, because the trailing edge Q2d easily flips up due to the higher degree of stiffness compared with plain paper Q1. In that case, because the reflected light R2 from the OHP sheet Q2 does not enter the light-receiving unit 102b before the trailing edge Q2d of the OHP sheet Q2 has arrived (downstream side in the sheet transportation direction Xd), the reflective optical sensor 102 falsely detects that the trailing edge Q2d has passed (detects at a time earlier than the detection time for the plain paper Q1) before the trailing edge Q2d of the OHP sheet Q2 passes through.
FIGS. 7A to 7F are timing charts illustrating detection timings for the sheets Q1 and Q2 by the reflective optical sensor 102, a start/stop timing for the rotational driving of the registration roller 101, and transportation timings for the sheets Q1 and Q2 by the registration roller 101 of the conventional sheet transport apparatus. FIGS. 7A to 7C illustrate timing charts for the case where the plain paper Q1 is transported, and FIGS. 7D to 7F illustrate timing charts for the case where the OHP sheet Q2 is transported. In FIGS. 7A to 7F, the reference letters Ts show a predetermined transportation time from the point where a change from sheet-present detection to sheet-absent detection is detected by the reflective optical sensor 102 to the stop of rotational driving of the registration roller 101. Furthermore, in FIGS. 7A to 7F, “reflective optical sensor ON” and “reflective optical sensor OFF” for the reflective optical sensor 102 illustrate “detection state—present” and “detection state—absent”, respectively, for the sheets Q1 and Q2; “registration roller ON” and “registration roller OFF” illustrate a “driving state” and a “driving stopped state”, respectively, of the registration roller; and “sheet transportation ON” and “sheet transportation OFF” illustrate a “transporting state” and a “non-transporting state” of the sheets Q1 and Q2 by the registration roller 101, respectively. The same applies for FIGS. 4A to 4G to be mentioned later.
When transporting the plain paper Q1 in the conventional sheet transport apparatus, as shown in FIGS. 7A to 7C, after the plain paper Q1 is transported by the registration roller 101 (after the trailing edge Q1d of the plain paper Q1 has passed through the registration roller 101), rotational driving of the registration roller stops (ref: α1 in the figure). On the other hand, when transporting the OHP sheet Q2, if false detection by the reflective optical sensor 102 occurs as shown in FIG. 6B, rotational driving of the registration roller stops (ref: α2) while the OHP sheet Q2 is still being transported by the registration roller 101 (before the trailing edge Q2d of the OHP sheet Q2 passes through the registration roller 101) as shown in FIGS. 7D to 7F.
Thus, in the conventional sheet transport apparatus, if false detection by the reflective optical sensor 102 occurs, it is determined that the trailing edge Q2d of the OHP sheet Q2 has passed through the registration roller 101 even if the OHP sheet Q2 is still being transported by the registration roller 101, and driving of the registration roller 101 is stopped while the OHP sheet Q2 is still being transported by the registration roller 101. At this time, if the OHP sheet Q2 has arrived at the nip region between the image carrier and the transfer member or the nip region between the intermediate transfer body and the transfer member, the registration roller 101 whose driving has been stopped while the OHP sheet Q2 is still being transported is forcefully rotated by the OHP sheet Q2, and a transportation load is applied to the OHP sheet Q2, and therefore pulling the OHP sheet Q2, thereby possibly causing image defects such as a dislocated transfer.
On the other hand, JP H6-87550A discloses a configuration in which a transmissive optical sensor is disposed so that its optical axis achieves a predetermined angle with respect to the sheet transportation face; however, in the case where the transmissive optical sensor is used in the sheet transport apparatus instead of the above-described reflective optical sensor as well, because the angle of the optical axis of the transmissive optical sensor with respect to the sheet transportation face differs depending on the characteristics of the sheet such as stiffness, as in the case of the sheet transport apparatus provided with the above-described reflective optical sensor, there is a possibility of images defects being caused such as dislocated transfer due to the occurrence of false detection of the trailing edge of the sheet.
Such inconveniences become particularly notable when the detection is performed by the optical sensor at a sheet flection forming portion (ref: reference numeral 75 in FIGS. 6A and 6B) for reliably flexing the sheet so as to arrange the leading edge of the sheet in parallel to the registration roller.